Assembly of immunoglobulin heavy chain (IgH) genes during B cell differentiation requires two recombination events. First, one of several DH gene segments rearrange to JH gene segments to produce DJH joins. Second, one of several hundred VH gene segments rearrange to the DJH join to produce V(D)J recombined alleles. A small proportion of V(D)J recombined alleles will encode IgH protein. Those cells that make IgH protein will terminate recombination of the IgH gene, a process referred to as allelic exclusion. Earlier studies indicate that allelic exclusion operates at the step of VH to DJH recombination. The mechanism by which VH recombination is stopped is not known. In recent studies of the chromatin structure of the IgH locus we found that VH genes are activated by three distinct mechanisms. The largest VH J558 gene family, that is at the 5' end of the locus, is activated by IL-7. The 3' -most VHS, represented by VH8 1X and VHSM7, are activated only in cells that contain DJH recombined alleles. Genes that lie between VHJ558 and VHSM7, represented by VH10, are activated in V-abl transformed cells suggesting that they may be activated by tyrosine kinases. In this application we propose to rigorously establish mechanisms that activate the intermediate and 3?-VH genes (Aim 1). We then propose to test the model that allelic exclusion is a consequence of terminating signals that activate VH genes (Aim 2). This will be accomplished by a combination of in-vitro and in-vivo analyses in genetically altered mouse strains. These studies are important not only for a comprehensive understanding of IgH gene assembly, but also for insights into the more general question of how large segments of the genome are activated.